This invention is related to a circuit for generating a gate bias for field effect transistors.
Field effect transistors require, in addition to an operating voltage applied between source and drain, another voltage which is applied between gate and substrate or between gate and channel or between gate and source or drain. For example, field effect transistors of the depletion type are usually constructed so that, besides an operating voltage which is applied between their source and drain, a voltage of opposite polarity to the operating voltage is applied between their gate and source. Thus, in such n-channel field effect transistors, a negative gate bias is needed besides a positive operating voltage. Such a negative gate bias may be supplied, for instance, via an additional external voltage source. Apart from an added cost, external voltage sources are not practical for various applications.
Another possibility is to generate from the given operating voltage a negative gate bias by charging energy storing elements by means of voltage transformers. Such generation of the gate bias is costly and therefore disadvantageous.
A negative gate bias can also be obtained by raising the source potential above ground, in consequence of which the source electrode has to be blocked against the reference ground high-frequency-effects with a capacitor. But the impedance situation for the high-frequency source grounding is altered considerably by the insertion of a capacitor, so that an unacceptable gate bias is generated for field effect transistors operating in a frequency range of about 3 to 5 GHz, such as gallium arsenide field effect transistors.
In many cases, direct source grounding is indispensable in microwave engineering so that a second, negative operating voltage to adjust the drain current becomes necessary. Adjusting or regulating the drain current as a function of the gate bias is of importance in field effect transistors in view of the requirement that the operating point of the field effect transistor is adequately controllable.